1. Field of the Invention
The present invention relates to a method of synthesizing trioxymethylene from formaldehyde by the catalytic action of an acidic ionic liquid.
2. Description of the Related Art
Paraformaldehyde products are widely used in a variety of industries such as car manufacture, machinery, electricity, electronics, instrument, agriculture, manufacture of building materials, light industry, etc., for their superior chemical stability, mechanical strength, and plasticity. Currently, two major methods are used to manufacture paraformaldehyde international-wide, the gaseous formaldehyde method and the trioxymethylene method. Copolymerization technique wherein trioxymethylene is used as polymerizing monomer accounts for 80% paraformaldehyde products all over the world, and therefore techniques for troxymethylene synthesis are essential to paraformaldehyde synthesis. In addition, trioxymethylene is not only useful in the manufacturing of paraformaldehyde resin, but also an important chemical material having a variety of uses such as in the preparation of anhydrous formaldehyde and pesticide, moulding material, bonding material, disinfectant agent, antibacterial agent, etc. Trioxymethylene can be applied to all reactions involving formaldehyde, and is especially useful in reactions employing anhydrous formaldehyde as a reactant.
Currently, sulfuric acid method is commonly used to synthesize trioxymethylene in the industry. However, defects such as high corrosivity of dilute sulfuric acid, high demands on devices used in the method, high cost, large amount of by-products, etc., exist. In 1978, Wells et. al (U.S. Pat. No. 4,110,298) employed polyethylene glycol monobutyl ether as dilutent and used two phase reaction catalyzed by sulfuric acid to produce trioxymethylene. However, the introduction of the third constituent also brought difficulties to the later isolation procedure. Acidic ion exchange resin was used and supported phosphoric acid and sulfuric acid were used as catalysts subsequently (DE-C-1 593 990 and AT-B 252 913, U.S. Pat. No. 5,962,702), however, relatively good result was achieved only with high concentration of formaldehyde and under increased pressure. In 1999, Kashihara et al. (U.S. Pat. No. 5,929,257) reported a constantly stable system of producing trioxymethylene from formalin, in which raw materials need to be pretreated. Yoshida (U.S. Pat. No. 4,381,397, U.S. Pat. No. 4,563,536, Emig (U.S. Pat. No. 5,508,448, U.S. Pat. No. 5,508,449) and Hoffmockel (U.S. Pat. No. 6,124,480) et al. reported respectively in 1983, 1986, 1996, and 2000 that heteropolyacid or supported heteropolyacid was used as catalyst to produce trioxymethylene. However, all these methods need a large amount of catalyst or have high requirement on reaction conditions. In China, GuanJian et al. reported, in Natural Gas Chemical Engineering (period 5, 2005), that supported PW12/ AC catalyst prepared by dipping active carbon as support in phosphotungstic acid was used to produce trioxymethylene, and the catalyst showed high activity with the relative content of trioxymethylene in gas phase of up to 30%. However, 70% of formaldehyde was used as raw material.
It has been reported that it is cost-effective to use protonic acids (such as sulfuric acid, p-tolyl sulfonic acid etc.) as catalyst to produce trioxymethylene. However, since the protonic acids are highly corrosive, difficult to separate, and cause contamination, zirconium needs to be used as corrosive resistant material on the equipments, which is impractical due to its high cost; when Lewis acid (such as zinc chloride, tantalum pentachloride, bismuth trichloride, titanium tetrachloride, tin tetrachloride etc.) or heteropolyacid or other solid acids are used as catalysts, problems arise such as need for large amount of catalyst, difficult to isolate, and high cost. Therefore, what is needed is a trioxymethylene synthesis system with high-efficiency and low corrosivity, which can achieve effective catalytic circulation, and lower costs of production and investment.